To determine the relative efficacy of two new experimental chemotherapeutic agents, 5-azacytidine and guanazole, in the treatment of advanced adult acute non-lymphocytic leukemia which is no longer responsive to conventional chemotherapeutic agents. Of the 18 patients who received 5-azacytidine, 6 achieved remission and 5 of these were complete (28%). Twelve patients received therapy with guanazole and only one achieved partial remission. Principal toxicities encountered with the 5-azacytidine therapy included gastro-intestinal intolerance, fever and neuromuscular toxicity. Fever was the principal toxicity observed after guanazole therapy. 5-azacytidine therefore has significant activity as an induction agent in ANLL, but guanazole appears to have no benefit for these patients.